1，event 21307096描述

下面描述来至ORACLE官方文档

The SCN delta in million units is with the range of values from 1 to 4095 which increases
the scn by:
lowest_scn + event level * 1000000
Example: if the lowest datafile checkpoint scn in the database is 990396
and the highest is 992660 then SCN delta is 1; given by (992660 - 990396) / 1000000
event="21307096 trace name context forever, level 1
Here are some tests in 12.1.0.2 using each level for alter database open resetlogs:
level 1 Elapsed: 00:01:02.35
level 2 Elapsed: 00:02:16.23
level 6 Elapsed: 00:06:08.05
In general: based on a 16k per second scn rate (16K/sec) , the open resetlogs time
would be at least (event level * 1000000 / 16000) seconds. Then level 1 would be at least
62+ seconds and level 4095 would be 71+ hours !.